The mold-immersion-type, vacuum, countergravity, shell mold casting process is particularly useful in the making of thin-walled, near-net-shape castings and involves: sealing a bottom-gated mold, having a gas-permeable upper portion, to the mouth of a vacuum chamber such that the chamber confronts the upper portion; immersing the underside of the mold in an underlying melt; and evacuating the chamber to draw melt up into the mold through one or more of the gates in the underside thereof. Such a process is shown in U.S. Pat. No. 4,340,108 wherein the mold comprises a resin-bonded-sand shell having an upper cope portion and a lower drag portion sealingly mounted to the mouth of the vacuum chamber by means of spring clips. U.S. Pat. No. 4,340,108 seals the mold to the vacuum chamber atop the cope such that the parting line between the mold halves lies outside the vacuum chamber. U.S. Pat. No. 4,632,171 seals the mold to the mouth of the vacuum chamber atop the drag such that the parting line between the cope and drag falls within the vacuum chamber. U.S. Pat. No. 4,658,880 mounts the mold to the vacuum chamber by means of a plurality of reciprocable and rotatable shafts having self-tapping threads on the lower ends thereof engaging mounting sites atop the mold. Chandley, G. D. Automatic Countergravity Casting of Shell Molds, Modern Casting, October 1983, pages 29-31, mounts round molds to a round vacuum chamber having self-tapping threads which screw into the periphery of the mold. U.S. patent application Ser. No. 147,863, filed Jan. 25, 1988 and assigned to the assignee of the present invention, mounts the mold to the vacuum chamber via a plurality of T-bar keepers engaging anchoring cavities in the mold.
The aforesaid references all disclose rigid vacuum boxes and molds whose upper and lower halves are glued together. The gluing process is expensive and time consuming and elimination thereof would improve the efficiency and economics of the process. Moreover, when the aforesaid mold-chamber arrangements are used with molds having more than about 400 square inches of mold confronting the vacuum chamber, there is a tendency for the molds to bow or flex into the chamber when the casting vacuum is drawn therein unless they are made extra strong/thick. This flexure can destroy the mold either by cracking or fracturing the mold or occasionally causing implosion thereof into the chamber.
Two techniques for eliminating gluing the mold portions together and reducing undesirable inward flexure of the mold are the subject of copending U.S. patent applications Ser. Nos. 211,024 and 211,023 filed concurrently herewith and assigned to the assignee of this application. These techniques provide substantially rigid means for pressing the mold portions together and resisting inward flexure of the mold. Such structures, however, do not accommodate process variations well. Hence variations in mold dimensions or untrue mating of the mold with the vacuum chamber can result in improper engagement between the mold and vacuum chamber and/or the creation of stress concentration sites which can cause cracking/fracture of the mold. Moreover, on an automated basis systems such as described in U.S. Ser. No. 211,023 require additional means for locating the pressers and controlling the amount of force applied thereby to prevent damage to the molds or dislodgment thereof from the mouth of the vacuum chamber. It would be desirable to eliminate such extraneous locating and control means and otherwise provide apparatus more tolerant of process variations.
It is the principal object of the present invention to provide an improved simple, self-adjusting apparatus for the vacuum, countergravity casting of unglued shell mold portions including means for resiliently biasing the upper mold portion into sealing engagement with the lower mold portion, resisting destructive flexure of the mold during casting and avoiding the creation of stress concentration sites in the assembly. This and other objects and advantages of the present invention will become more readily apparent from the detailed description thereof which follows.